In recent years, recording apparatuses and the like using rotating discs have had an increase in a memory capacity and an increase in a transfer rate for data. Thus, bearings used for such a recording apparatus is required to have high performance and high reliability to constantly rotate a disc load with control of a high accuracy. Accordingly, hydrodynamic bearings suitable for high-speed rotation are used for such rotary devices.
The hydrodynamic bearing type rotary device has a lubricant such as oil between a shaft and a sleeve, and generates a pumping pressure by hydrodynamic grooves during rotation. Thus, the shaft and the sleeve rotate in a non-contact state with respect to each other in the hydrodynamic bearing type rotary device so it is suitable for high-speed rotation.
Hereinafter, an example of conventional hydrodynamic bearing type rotary devices will be described with reference to FIG. 15. As shown in FIG. 15, a conventional hydrodynamic bearing type rotary device includes a shaft 21, a flange 22, oil 24, an upper cover 25, a hub 27, and a base 28.
The shaft 21 is integral with the flange 22. The shaft 21 is inserted into (fitted to) a bearing hole 23A of a sleeve 23 so as to be rotatable relative to the sleeve, with a gap G11 in a radial direction being interposed therebetween. The flange 22 opposes a lower surface 23C of the sleeve 23 and forms a bearing surface having a gap S11. The flange 22 also has a clearance portion (different dimension portion) 22B having a gap S12 on an inner peripheral side. On at least one of an outer peripheral surface of the shaft 21 and an inner peripheral surface of the sleeve 23, radial hydrodynamic grooves 23B are formed. On at least one of the sleeve lower surface 23C and an upper surface of the flange 22, thrust hydrodynamic grooves 22A are formed. The upper cover 25 is fixed to the sleeve 23 or the hub 27 having a gap S13 to the sleeve 23. Between an inner periphery of the upper cover 25 and an outer periphery of the shaft 21, a gap G13 in the radial direction is formed. Between an outer periphery of the flange 22 and an inner peripheral surface of the hub, a gap G12 in the radial direction is formed.
The clearance portion 22B having the gap S12 is not always necessary for performing a function as a bearing. However, as the motors have been miniaturized, the clearance portion is often provided in order to secure a predetermined floating level without increasing bearing loss. When the thrust hydrodynamic grooves 22A are in a spiral pattern, which is well known in the art as a pattern which generates pressures toward the inner periphery, a pressure becomes larger toward the inner periphery along the thrust hydrodynamic grooves 22A. The pressure does not decrease even when it comes near the clearance portion 22B and is maintained at a high level. Since the clearance causes the thrust gap to spread equivalently, the bearing loss can be reduced. In this way, a predetermined floating level can be secured without changing a rotational rate and/or weight of the motor.
The flange 22 and the gap S13 of the upper cover 25 communicate with one another by a communication hole 23E. At least the oil 24 is filled or held in the bearing gaps near the hydrodynamic grooves 23B and 22A and the communication hole 23E. To the hub 27, a disc 29 is fixed. To the base 28, the shaft 21 is fixed. A rotor magnet 30 is also fixed to the hub 27. A motor stator (not shown) is also fixed to the base 28 at a position opposing an outer periphery of the rotor magnet 30. If the base 28 is made of a magnetic material, the rotor magnet 30 generates an attraction force in an axial direction by leakage flux and presses the sleeve 23 toward the flange 22 with a force of about 10 to 50 grams. If the base 28 is not made of a magnetic material, an attraction plate made of a magnetic material having a ring shape is fixed to the base 28 at a position opposing an end surface of the rotor magnet 30).
Now, operations of the conventional hydrodynamic bearing type rotary device as described above will be described with reference to FIG. 15. In the above conventional hydrodynamic bearing type rotary device, when a rotational force is applied to the rotor magnet 30 by an electromagnetic function with the motor stator, the hub 27, the sleeve 23, the upper cover 25, and the disc 29 start to rotate. When these members rotate, the hydrodynamic grooves 23B and 22A gather the lubricant 24 such as oil to generate pumping pressures between the shaft 21 and the sleeve 23, and between the flange 22 and the sleeve 23. In this way, the rotating part including the sleeve 23 rotate in a non-contact state with respect to the shaft 21 and the flange 22. With a magnetic head or an optical head (not shown), data can be recorded/reproduced to/from the disc 29.